In mobile communications, the transmission conditions change over time and space. Therefore, for a base station (BS) to properly schedule the transmission of data to an item of user equipment (UE), using appropriate modulation and code rates, the UE regularly reports channel conditions to the BS. In one approach to sending channel state information (CSI), the UE sends regular Channel Quality Indicator (CQI) reports. The CQI is an index value that efficiently quantizes measured signal quality at the UE.
Of course, there may be many UEs hosted on the same BS at the same time, and all UEs need to send CQI reports. Further, each UE may have multiple CQI values to report. For example, CQI reporting for an Orthogonal Frequency Division Multiplex (OFDM) downlink signal may require UEs to report wideband CQI values, along with one or more narrowband CQI values that are restricted to particular frequency sub-bands of interest. As such, CQI reports must contain as few bits as possible.
One approach to reducing the number of bits needed to report multiple CQI values adopts a “differential” reporting approach. With differential reporting, one or more CQI values are reported relative to one or more other CQI values. Long Term Evolution (LTE), as standardized by the Third Generation Partnership Project (3GPP) provides a specific example of differential reporting. Assuming LTE MIMO (Multiple-Input-Multiple-Output) transmission, an example CQI report from an LTE UE to an eNodeB includes one or more of the following items: a rank indicator (RI), a CQI value for the whole bandwidth, CQI values for parts of the bandwidth, and Precoding Matrix Indexes (PMIs), to guide transmission precoding by the BS.
The “rank” of a MIMO channel defines how many layers can be used in the transmission of data streams from a MIMO transmitter to a MIMO receiver, and the maximum rank is given as min (NTX, NRX). Here, “N” represents the number of transmit antennas (TX) or receive antennas (RX). UEs use the RI to indicate a rank preference, which the eNodeB may use in determining the appropriate channel rank to use for transmissions to the UE.
Currently, the LTE standards use a baseline antenna configuration of 2×2, which denotes two transmit antennas and two receive antennas, and provide for a maximum of two “codewords.” Codewords are coded bits that are mapped to modulation symbols, which are then mapped to one or more layers. Each codeword thus may be understood as a different data stream, where the two codewords may have different modulation and coding schemes applied to them. Further, according to the (current) LTE standards, each codeword may use up to two layers in its transmission.
Thus, a UE receiving LTE MIMO transmissions on the downlink reports a CQI value for each codeword, where the CQI value reported for one of the codewords represents a differential value that specifies a limited-range offset relative to the CQI value reported for the other codeword. In more detail, assume a CQI reporting mode of “PUCCH 1-1” and a “Rank 2” transmission. (The “PUCCH 1-1” mode denotes CQI reporting by the UE on the Physical Uplink Control Channel (PUCCH), for Frequency Division Duplexing (FDD), assuming AWGN channel conditions.) In this context, the UE reports a 4-bit CQI value for the first codeword (CW1) and a 3 bit-differential CQI for the second codeword (CW2).
The differential CQI is defined as the CQI value determined for the second codeword (CQI2), as constrained by the differential offset limit of [−4, 3]. In other words, the CQI value for the second codeword is reported relative to the CQI value for the first codeword using three bits, instead of four bits. Therefore, CQI2 can take on only eight index values falling within the stipulated offset interval [−4, 3]. For more information regarding this formulation, and CQI reporting in general, within the LTE context, please refer to the following two documents: 3GPP 36.213, “Physical layer procedures”, V9.1.0 (2010); and 3GPP 26.211, “Physical Channels and Modulation”, V9.1.0 (2010).